1. Field of the Invention
The present invention relates to MRI (magnetic resonance imaging) apparatus and method which excite nuclear spins of an object magnetically with an RF (radio frequency) signal having a B1 magnetic field at the Larmor frequency and which reconstruct an image based on NMR (nuclear magnetic resonance) signals responsively generated due to the excitation, and more particularly, to MRI apparatus and method which make it possible to reduce influence of B1 inhomogeneity by universal B1 shimming without depending on characteristics of each object.
2. Description of Related Art
Magnetic Resonance Imaging (MRI) is an imaging method which excites nuclear spins of an object set in a static magnetic field with an RF magnetic signal B1 having the Larmor frequency and then reconstructs an image based on received NMR signals generated due to the excitation.
In recent years, B1 inhomogeneity has presented a problem that increases with higher static field MRI apparatus. B1 inhomogeneity is also called RF magnetic field inhomogeneity. B1 inhomogeneity is a phenomenon where the degree of heterogeneity is increased due to attenuation of a higher frequency excitation RF pulse with a shorter wavelength in a living body and where the RF echo-response signal may also become inhomogeneous in a higher magnetic field apparatus requiring a higher resonant frequency. That is, in a high magnetic field MRI apparatus, signal intensity distribution of an RF transmission pulse varies widely between a center and a peripheral part of an imaged area due to the influence of a difference in electric permittivity between imaged objects and the like.
Therefore, B1 shimming is required for reducing the influence of B1 inhomogeneity. That is, if a correction to equalize signal intensity distribution of an RF transmitted pulse is not performed, nonuniformity in sensitivity may be generated between the central part and the peripheral part of an imaged area. For that problem, research shows that transmitting the RF transmission pulse with a corrected amplitude and phase in an area to generate nonuniformity in sensitivity is effective for reducing the influence of B1 inhomogeneity (for example, refer to G. McKinnon et al., “RF Shimming With a Conventional 3T Body Coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p173, D. Weyers et al., “Shading Reduction at 3.0T using an Elliptical Drive”, Proc. Intl. Soc. Mag. Reson. Med. 14 (2006), p2023 and J. Nistler et al., “Homogeneity Improvement Using A 2 Port Birdcage coil”, Proc. Intl. Soc. Mag. Reson. Med. 15 (2007), p1063).
However, an area where sensitivity nonuniformity is generated by an influence of B1 inhomogeneity and an appropriate phase and amplitude of an RF transmission pulse vary depending on each characteristic such as a weight, a height, a volume, a water distribution, a fat distribution and a muscle distribution of an object. To the contrary, B1 shimming proposed conventionally is the method to obtain the phase and the amplitude of the transmission RF pulse appropriate for the characteristics of a specific object in advance and to transmit the transmission RF pulse having the determined phase and amplitude to the area where the sensitivity nonuniformity is generated depending on the characteristics of the object. Therefore, appropriate B1 shimming cannot be performed constantly since the sensitivity nonuniformity area and an appropriate amplitude and a phase of RF transmission pulse also change when the object changes. In other words, when the conventional method is used as it is, the amplitude and the phase of the transmission RF pulse are adjusted regarding the common sensitivity nonuniformity area determined in advance though the sensitivity nonuniformity area varies depending on each object.
Further, it is necessary for a user to obtain an appropriate phase and amplitude of transmission RF pulse and to perform adjustments of the phase and the amplitude of the transmission RF pulse manually and frequently every time the object changes.
The problem mentioned above hinders implementation of B1 shimming function in a high magnetic field apparatus. For that reason, developing a technology for universal and simple B1 shimming coping with changing the object is desired.